1. Field of the Invention
This invention relates to a method of controlling diaphragm driving in an electronic still camera and a device therefor, and more particularly to a method of controlling diaphragm driving during photographing in a movie mode of an electronic still camera and a device therefor.
2. Description of the Related Art
Recently, there has been developed an electronic still camera wherein an image pickup device such as a solid state image sensing device or an image pickup tube is combined with a recording device using a magnetic disc which is an inexpensive rotary recording medium and yet has a comparatively high memory capacity, a subject is still-photographed pure-electronically and the photograph recorded into a rotating magnetic medium, and the reproduction of an image is carried out by a television, a printer and the like, which are provided separately.
In this electronic still camera, through the utilization of characteristics of an image sensing device such as a CCD(Charge Coupled Device), a movie mode can be used, in which the electric charge of the image sensing device is read out and so on to obtain images continuously, with a shutter being opened.
In the electronic still camera of the type described, an open photometric system is adopted because of its ability to frame and to measure distance easily in the same manner as in the conventional so-called silver salt still camera. In this case, in the still mode where still photographing is carried out, it is necessary to instanteously stop down from an open position to reach to a proper exposure value calculated on the basis of the photometric data. Because of this, in the still mode, high response in the control of diaphragm driving is required.
In contrast thereon, in photographing in the movie mode, the photometry and the control of diaphragm driving are carried out in a state where stop-down is performed on the basis of the results of photometry. The exposure value may be compensated by a value of variations in brightness as compared with the photographing in the still mode, thus enabling to reduce the value of action of driving the diaphragm in general.
In the conventional electronic still camera of the type described, as to the control of the diaphragm driving, an insensitive zone of a suitable width is provided to the proper exposure value obtained from the ressults of photometry of the subject, and, when an actual exposure value is within the range of the insensitive zone, the diaphragm driving is brought to a stop. This control method is simple, whereby the method is used widely in general, and, when the accuracy of photometry is good and stabilized, the method proves to be effective. However, in the photographing in the still mode, high speed response should be made to the result of photometry. If the control system is constructed in this way, then the diaphragm driving system gives a sharp response to the result of photometry, whereby, in the case of photographing in the movie mode, hunting is resulted. Accordingly, in order to cope with the photographing of both the still mode and the movie mode, the aforesaid insensitive zone should necessarily be widened.
When the width of the insensitive zone provided for the proper exposure value is widened as described above, although the accuracy may be lowered, hunting does not occur during photographing, the system becomes strong against noises and suitable for the photographing in the movie mode. However, such a problem is presented that the still camera cannot cope with the photographing (still photographing) in the still mode, which is the primary function of the electronic still camera, because of lowered accuracy.